This invention relates generally to a closure assembly for containers and more particularly to such assemblies for use with evacuated blood collection tubes such as VACUTAINER.RTM. Brand evacuated tubes. More particularly, this invention relates to such a closure assembly and the deliberate profile formation of the rubber or elastomer stopper portion of the assembly generated from a computer readout in order to eliminate tensile stress along the stopper/ container wall interface so as to more clearly define and seal the container with the stopper. The computer generated stopper of the invention is so configured that the forces generated by the insertion of the stopper into the container are utilized in order to improve and develop continuous compressive radial stresses along the entire stopper/container interface.
As is well known in the medical field, both evacuated and non-evacuated tubes are used in large quantities to take blood samples from patients for subsequent testing of the blood for various purposes to determine if a patient has certain diseases or blood problems or other physical health problems of some kind. It is routine for a laboratory technician, for example, to take several such samples from a single tube for various tests. Since the tube may be evacuated, there is a pressure differential across the stopper holding the sample in the tube. Thus, when a needle is inserted through the diaphragm of the stopper, there can be an aerosol of the sample which may spray onto the technician. Moreover, when taking samples, sometimes blood droplets are left on the top and bottom surface of the stopper during the removal of the needle once the sample has been taken from the evacuated tube which creates a contamination problem for anyone handling the stopper subsequently.
A further problem with the use of stoppers of the kind discussed above, is the fact that an improper seal is generated along the stopper/container surface interface. The seal is discontinuous merely because of the stresses generated by the configuration of the stopper itself, and its interaction with the container involved. Such problems have the effect of reducing the shelf-life of evacuated tubes and are referred to by practitioners-in-the-art as "gray band" regions because they look gray when looking at the stopper through a transparent container wall. When one realizes the vast number of such tubes required for daily taking of blood samples in many many hospitals, it becomes important to reduce the cost of such items by increasing the shelf-life thereof. If a vacuum can be maintained at a proper level for a longer period, it follows that the evacuated tubes will have a much longer shelf-life and be in a better condition for use when required.
Attempts have been made in the past to overcome some or all of the above-noted problems. For example, U.S. Pat. No. 3,974,930 to Gizard et al. teaches a stopper with a central hole in the top surface of the diaphragm for protecting personnel from blood droplets on the outside surface of the stopper diaphragm. However, the Gizard et al. stopper uses a flat diaphragm surface on the internal surface facing the inside of the container which will permit "red spot" formation on that surface. This phenomenon is caused by the development of a conical break in the diaphragm bottom wall upon insertion of a needle. Thus, when the needle is removed, a red spot of blood develops in the break on the internal or bottom surface of the diaphragm. Thus, if a clinician removes the stopper for taking a sample, the bottom surface of the stopper is contaminated and may cause contamination by the clinician touching the bottom surface of the stopper.
E. P. Percarpio, in his U.S. Pat. Nos. 4,301,936; 4,187,952; 4,186,840; 4,136,794; and 4,111,326, teaches a method for reducing the force required to assemble the stopper in the appropriate tube, while achieving, at the same time, a satisfactory sealing characteristic. The patents describe diaphragms in the stoppers disclosed comprised of curved upper and lower surfaces with a constant diaphragm thickness of 0.04-0.09 inches, with the point being to reduce the thickness of the diaphragm and reduce the effort required to insert the needle through the diaphragm into the tube. There is no recognition in these patents of generating a stopper profile so as to eliminate any tensile stress along the stopper/container interface and to develop complete compressive radial stress at the stopper/container interface.
With this invention, by contrast, a three-dimensional, axisymmetrical computer model has been developed for a stopper for use in, for example, a conventional 13 millimeter diameter evacuated tube. A linear Young's modulus of 500 p.s.i. was used together with a poissons ratio of 0.495. The profile generated, allows for a proper configuration of stopper with appropriate upper and lower surface profiles in order to eliminate any tensile stress along the container/stopper interface.
In connection with generating the desired profile, it is necessary prior to generation to develop the appropriate boundary conditions in order to guide the computer generation. For example, it is important to establish fixed nodal locations in order to identify the region generated. Such positions include for example, points along the bottom surface of the upper annular flange of the stopper which are to remain, in a resulting generated stopper, fixed at the mating top surface of a container. These points do not move in the Z axis direction, and thus define boundaries for the desired profile.
Moreover, loading conditions must be selected for the profile. This is done by selecting a desired container and the desired selected stopper. From this, the nodal points along the tapered surface of the stopper profile which are to meet and conform to a container surface are defined. The distance of the movement of these points of the stopper in the Y axis from the position of the points in the stopper prior to insertion, and the position after insertion in the container are determined for selecting boundary and/or displacement conditions. Thus, a model stopper, so-to-speak, is selected which determines boundary and loading conditions in order to obtain the desired profile.
It should be borne in mind that this invention is directed not only to a stopper for use with blood collection tubes and including both evacuated and non-evacuated blood collection tubes, but also this invention is directed to a stopper cap assembly much like that taught and claimed in U.S. Pat. No. 4,465,200 which is hereby incorporated by reference in its entirety. In that patent, an arrangement is provided for reducing to a minimum the exposure of aerosol or blood droplets which may be evident in removing a blood sample from an evacuated tube on the top surface of the assembly. A cap is provided in combination with the stopper of the assembly which is mounted over the resilient rubber stopper of the evacuated tube. The cap includes a top portion which extends over the top of the stopper to define between the top surface of the stopper and the bottom surface of the extended top portion of the cap a space which serves to contain any blood droplets or aerosol generated by needle insertion and withdrawal.
This patent, while teaching an assembly which is effective for protecting against blood droplets or aerosol on the top surface of the assembly, has no effect upon any such collection of blood droplets which may form on the bottom surface of the diaphragm of the elastomer stopper of the assembly. Thus, as discussed above, the invention here includes a generated stopper profile which eliminates "red spot" on the bottom surface of the diaphragm thereof as well as developing continuous compressive radial stress along the entire stopper/container interface.
With the foregoing and additional objects in view, this invention will now be described in more detail, and other objects and advantages thereof will be apparent from the following description, the accompanying drawings, and the appended claims.